Aggressive device scaling and interconnection ground rules are challenging the physical limits of materials, processes, and structures in the semiconductor industry. For wiring patterns formed on semiconductor chips and packages, copper has emerged as the metallurgy of choice because of various beneficial properties. Despite the various advantages of copper, the electromigration lifetime of a copper film depends strongly on the processes used to form the copper film. For example, the activation energy required to create failures, due to electromigration of a copper film, typically ranges from 0.7 to 1.0 eV. It is desirable to produce a copper-containing film in which the activation energy required to cause failure is increased beyond 1.3 ev. It is further desirable to produce such a film without major processing modifications and without the addition of multiple processing steps. It is also desirable to produce such a film without bringing about any performance degradation.
The thermo-mechanical, electrical, and metallurgical properties, microstructure, and etching characteristics of a film depend on the process used to produce the film. More specifically, these qualities depend upon the microstructure of the metal film so produced. The microstructure of the metal film is enhanced when dopant impurity materials are disposed along the grain boundaries of the film. These impurities help to suppress grain growth and grain recovery within the film. Uncontrolled grain recovery and grain growth may cause defects during subsequent processes, in addition to compromising the qualities noted above.
The presence of partially soluble and insoluble intermetallic materials within a heat-treated metal film produce a microstructure which includes a high twinning density (multiple twins per grain). These intermetallic materials will be preferentially segregated along grain boundary regions and near the surface of the copper-containing film. The presence of these partially soluble and insoluble intermetallic materials, along copper grain boundaries and near the copper surface, reduces copper grain boundary mobility and the mobility of copper atoms along the surface. The interaction of the impurities and the high twinning density formed within the copper microstructure enhances the electromigration lifetime of the entire film structure being used as an interconnect material. This enhancement occurs because such a structure requires more energy to cause atomic migration preferentially in any given direction. Electromigration failures happen when significant atomic migration occurs preferentially in one direction.
What is needed is an improved process and structure, for producing a copper film used as a wiring interconnection material, offering increased resistance to electromigration failures.